I think it is a misconception that Sam Colt was left-handed. His revolvers were made during the time of cavalry soldiers. The cavalry soldier's primary weapon was his saber. The saber was carried on his left side to be drawn with the right hand.

That would leave his revolver as a secondary weapon to be carried on his right side with the butt forward. He could then draw it with his left hand or his right. I believe this is how they were trained.

Now with the revolver in his left hand, as it would most likely be, the soldier could recap the nipples with his right hand. This requires a little more dexterity. I contend that this is why Colt put the nipple cutout on the right side. It just carried over into the later model Colt Peacemaker, with the loading gate on the right side.